Stunted Animals Found in Forest Fragments

Mice and shrews living in isolated remnants of old-growth rainforest are smaller than their counterparts in large tracts of pristine forest. A study of five mammal species on the Olympic Peninsula in western United States found that three species had differences in body weight and length that related to the size of forest they lived in.

The research compared animals from isolated forest patches, less than 150 acres (60 hectares) in area, with those inhabiting clearcuts, second-growth forests and old-growth forests left in large expanses or in long corridors. Changes in body size arose over a surprisingly short period of time. The fragments of forest studied were severed from large forests just 30 years earlier.

Two species were consistent in size throughout the range of habitats: common deer mice (Peromyscus maniculatus) and southern red-backed voles (Clethrionomys gapperi).

Three other species also reached the same size in both old-growth forest corridors and in large areas of old-growth forest: northwestern deer mouse (also called Keen's mouse, P. keeni), montane shrew (or dusky shrew, Sorex monticolus), and Trowbridge's shrew (S. trowbridgii). But compared to those forests, these same species in forest fragments were significantly lighter and shorter.

For instance, montane shrews in fragments averaged less than five grams in weight, while those from clearcuts, old-growth corridors and large forests weighed close to six grams. Similarly, Trowbridge's shrews in fragments grew to just over 5.8 centimetres long, whereas those from all other habitats were around 6.3 cm.

The fragments of intact rainforest are in effect islands of habitat. The findings in this study, however, counter the "island rule", a principle predicting that small animals evolve into larger creatures when they're isolated on tiny islands.

This research did not investigate how the transformations came about. The size discrepancies might be genetically based or result from environmental influences on individual condition. The stunted growth has widespread implications, though, since body size influences nearly all physiological and ecological aspects of an animal's existence.